Definitions of subtle:

• adjective:   able to make fine distinctions

  Example: "A subtle mind"

• adjective:   faint and difficult to analyze

  Example: "Subtle aromas"

• adjective:   be difficult to detect or grasp by the mind

  Example: "His whole attitude had undergone a subtle change"

• adjective:   working or spreading in a hidden and usually injurious way

  Example: "A subtle poison"